1. Field of the Invention
The present invention relates to an optical information recording and/or playbacking system and an optical head apparatus used in this system, especially used in an optical disk apparatus such as a compact disc, and further relates to a method for recording and/or playbacking optical information.
The present invention also relates to a technical concept in which recording to and playback from an optical medium, such as an optical disk having a first and/or a second recording layer which are formed inside or on one surface of a transparent substrate, wherein a first distance formed between a surface of the transparent substrate and the first recording layer is different from a second distance formed between the surface of the transparent substrate and the second recording layer, are performed.
2. Description of Related Art
In a conventional optical information recording and/or playbacking apparatus such as an optical disk apparatus, an optical medium, for example, an optical disk is formed from an optical information recording layer in the order of microns, and a transparent substrate is provided to protect the recording layer, and a minute spot of light is formed onto a surface of the recording layer by transmission through the transparent substrate to perform recording and playback.
In such an optical information recording and/or playbacking apparatus, a focused beam of light passes within a transparent substrate, which is a flat and parallel plate, and there occurs a wavefront aberration in the transmitted beam that exhibits a dependency on the thickness of the flat parallel plate.
This wavefront aberration manifests itself as a skew in the equiphase surface of the focused beam from being a truly spherical surface.
If this skew is large, because a minute spot that is close to the diffraction limit is formed from the focused light onto the surface of the recording layer, the lens in the optical system of the optical head mechanism is designed so as to correct this wavefront aberration.
Recently, it has been possible to include correction mechanism such as provided by a single aspherical objective lens.
The thickness of the substrate of an optical information recording medium differs, depending on the purpose thereof. When there is a difference in the substrate thickness, because the above-noted wavefront aberration amount of compensation also differs, in an optical head in the past, it was only possible to perform recording and playback with respect to an optical disk having a specific substrate thickness.
In contrast to the above situation, as noted in pages 27 through 29 of No. 1, Vol. 1 of the Optical Review (1994), there has been a proposal for using the 0th order transmitted light and 1st order diffracted light each being created by a hologram element to enable the use of a single optical head for recording and playback with respect to two optical disks having different substrate thicknesses.
FIG. 5 shows a general view of the related optical head apparatus.
The optical head apparatus which is shown in FIG. 5 is formed by a semiconductor laser 1, a hologram element 2, and an optical head comprising an objective lens 8, the objective lens 8 being designed so as to accommodate a first optical disk 11-1 which has a substrate thickness of 0.6 mm, playback being made from a disk having this thickness by means of the 0th order transmitted light of a hologram element 2, in which a change of phase does not occur.
To accommodate a second disk 11-2 having a substrate thickness of 1.2 mm the hologram element 2 has a grating formed so that 1st order diffracted light will correct the aberration which is dependent upon the transmitted light of the objective lens, in accordance with the difference in substrate thickness.
The shape of the grating of the hologram element is established by the interference of the incident light wavefront and the exiting light wavefront at the hologram surface position.
The optical medium such as an optical disk even as shown in FIG. 5, is also constructed from a recording layer and a transparent substrate, therefore the aberration which is caused by the parallel flat plate that is the transparent substrate, does not represent a simple spherical wave, but rather a complex aberration which includes an aspherical surface.
Therefore, the hologram element used to correct this type of aberration must also have a grating that in general forms an aspherical wavefront.
In view of the above-noted prior art, an object of the present invention is to provide an optical information recording and/or playbacking system and an optical head apparatus used in this system, especially used in an optical disk apparatus such as for a compact disc, and further provide a method for recording and/or playbacking optical information.
More specifically, another object of the present invention is to provide an optical information recording and/or playbacking system and an optical head apparatus used in this system, which are capable of accommodating any one of optical information recording mediums such as disks having a plurality of different substrate thicknesses.